rigidbody2D.velocity.y not working

I'm really new to Unity and want to add a sprite and move it by press "left" and "right" key.

First I add a RigidBody component to the sprite, then add a Script Component.

Following is the code of script file:

But after I pressed the left key, "left" string is outputted to the console, but the sprite never moved. After a lot of trying, I found it's because I used "RigidBody" component but not "RigidBody2D" component for the sprite. What a foolish mistake..